The present invention relates to a desulfurization method and a desulfurization system (desulfurization equipment) for removing sulfur oxides in flue gas and, more particularly, to a desulfurization method and a desulfurization system that enhance the desulfurization performance of a desulfurization absorber and contribute to a decrease in operation power for the whole system.
In a flue gas desulfurization system, as a raw material for limestone slurry used for a desulfurization absorber, rock-like limestone or limestone powder prepared already by grinding at a mine etc. is used. Usually, in Japan, flue gas desulfurization systems are often operated by using ground limestone powder as the raw material.
For example, in Japan, a lime-gypsum process flue gas desulfurizer uses slurry containing limestone powder as an absorbent for sulfur dioxide. As the particle size of limestone powder in absorbent slurry decreases and the dissolution rate increases, the limestone powder dissolves faster and the alkalinity of absorbent increases, so that the removal ratio of sulfur dioxide gas, which is an acid gas, increases. The reason for this is that acid-alkali reaction takes place in the desulfurization absorber, so that as the alkalinity of absorbent increases, the absorbent becomes easier to introduce acid.
On the other hand, grinding for decreasing limestone size requires power, and the power consumes larger amounts of energy as the particle size of ground powder decreases. Conventionally, therefore, the power for grinding limestone and the power for operating the desulfurizer have been compared, and as the result, limestone having an average particle size of about 10 to 20 xcexcm, for example, has been used.
However, there has been a problem in that a high circulation flow rate of absorbent in the desulfurization absorber is needed to achieve a desired removal ratio of sulfur dioxide.
On the other hand, the limestone powder has a fixed particle size distribution. Even if the average particle size is 10 to 20 xcexcm, not all particles exist in this range, and the particle size has a fixed distribution. It is mainly a large-particle component in the particle size distribution that exerts an adverse influence of limestone in the desulfurizer. Specifically, 90 to 95% of limestone put into the desulfurizer dissolves in the absorbent, and the remaining 5 to 10% of limestone exists in the absorbent and acts as a solid.
Thereupon, it is disadvantageous in terms of power to grind all limestone into a powder form. In the case where all particles are made fine, if the powder is made finer than 10 xcexcm, there arises a problem in that extremely high power is required for grinding. In this case, power far higher than the power for operating the flue gas desulfurization system is required for grinding.
To solve the above problems, the inventors conducted studies earnestly to develop a desulfurization method in which the concentration of limestone in a desulfurization absorber is increased while the concentration of residual limestone in by-product gypsum is kept low, by which the desulfurization performance of the absorber can be increased, and the operation power for the whole system can be reduced.
As the result of the studies, the inventors found that in a flue gas desulfurization method in which desulfurization is accomplished by bringing absorbent slurry containing limestone into contact with flue gas, large particles in the limestone slurry are ground into fine particles at the preceding stage of the desulfurization absorber, by which the aforementioned problems are solved. The present invention has been completed from this point of view.
The present invention provides a method for desulfurizing flue gas in which desulfurization is effected by bringing absorbent slurry containing limestone into contact with flue gas, comprising the steps of classifying absorbent slurry extracted from a desulfurization absorber, returning a fine-side fluid containing much limestone to the desulfurization absorber, and sending a coarse-side fluid containing much gypsum to a solid-liquid separator; mixing supplied limestone powder to yield limestone slurry by a solid-liquid mixer or supplying supplied limestone to a limestone grinder to yield limestone slurry; classifying the limestone slurry, sending the small-particle fluid component thereof to the desulfurization absorber, and sending the large-particle fluid component thereof to a limestone fine grinder; and finely grinding limestone contained in the large-particle fluid component by the limestone fine grinder and sending the finely ground limestone to the desulfurization absorber.
In the above-described method for desulfurizing flue gas, there can be used a mode in which some of filtrate obtained by the solid-liquid separator is sent to the solid-liquid mixer or limestone grinder to mix with limestone powder or limestone, or a mode in which the absorbent slurry extracted from the desulfurization absorber is sent to a gypsum dissolution tank after being thickened, and is classified after make-up water is supplied to the gypsum dissolution tank.
Also, the present invention provides a system for desulfurizing flue gas in which desulfurization is effected by bringing absorbent slurry into contact with flue gas, comprising a desulfurization absorber for desulfurizing flue gas by means of circulation of absorbent slurry; an absorbent slurry classifier for classifying absorbent slurry extracted from the desulfurization absorber and sending a fine-side fluid to the desulfurization absorber;
a solid-liquid separator for separating a coarse-side fluid separated by the absorbent slurry classifier into a gypsum component and a filtrate component; a solid-liquid mixer for mixing supplied limestone powder to yield limestone slurry, or a limestone grinder for grinding supplied limestone to yield limestone slurry; a limestone slurry classifier for classifying the limestone slurry into a small-particle fluid component sent to the desulfurization absorber and a large-particle fluid component subjected to fine grinding; and a limestone fine grinder for finely grinding the large-particle fluid component and sending the finely ground component to the desulfurization absorber.
In the above-described system for desulfurizing flue gas, there can be used a mode in which some of filtrate obtained by the solid-liquid separator is sent to the solid-liquid mixer or limestone grinder to mix with limestone powder or limestone, or a mode in which a thickener and a gypsum dissolution tank to which make-up water is supplied are further provided at the preceding stage of the absorbent slurry classifier for sending absorbent slurry extracted from the desulfurization absorber.
In the present invention, two classifying steps, a step of classifying absorbent slurry extracted from the desulfurization absorber (absorbent slurry classifier) and a step for classifying limestone slurry obtained by grinding limestone of raw material (limestone slurry classifier), are combined.
In the absorbent slurry classifier, after absorbent slurry is classified, the fine-side fluid containing much limestone is returned to the desulfurization absorber, and the coarse-side fluid containing much gypsum is sent to the solid-liquid separator. On the other hand, in the limestone slurry classifier, after the limestone slurry mixed with filtrate or make-up water is classified, the small-particle fluid component is sent to the desulfurization absorber, and the large-particle fluid component is set to the limestone fine grinder.
In particular, in the present invention, only large particles in limestone slurry is separated by the limestone slurry classifier (cyclone etc.) and is charged into the limestone fine grinder to be finely grounded. The limestone slurry in which large particles have been ground and removed is charged into the desulfurization absorber after being mixed with slurry. Thus, large particles have been removed from the limestone slurry introduced into the desulfurization absorber, so that the operation efficiency of the desulfurization absorber can be improved, and the power for the desulfurization absorber can be decreased. Also, the classifying operation of the absorbent slurry classifier can be enhanced, and the purity of the obtained gypsum can be made higher.
As described above, according to the present invention, only coarse limestone is classified on the downstream side of the limestone grinder and is recirculated to the grinder, so that excess grinding power is not needed, whereby the power of grinder can be decreased significantly.
Also, since coarse limestone can be prevented from being supplied to the desulfurization absorber, in the absorbent slurry classifier, the separation efficiency between gypsum, which is a by-product in the absorber, and residual limestone is improved dramatically. As a result, the concentration of limestone in the absorber can be increased while the concentration of residual limestone in the by-product gypsum is kept low. Since the limestone is made fine powder, the dissolution rate is increased, and the desulfurization performance of absorber is enhanced.
Hereinafter, the present invention will be described in detail by way of embodiments. The scope of the present invention is not subject to any restriction by the embodiments.